Polyphase sawyer motor forcer

ABSTRACT

An improved hybrid variable reluctance linear forcer design of the type known as a Sawyer motor forcer is provided with a 6-phase magnetic design. The 6-phase magnetic design has reduced force ripple compared to existing 2/4-phase and 3-phase designs providing smoother motion with lower velocity ripple. Three identical electromagnets each having four salient toothed poles with each pole being spaced at a unique 30 degree phase increment form a 12-pulse force function equivalent to a 6-phase motor. The electromagnets are provided with split-phase windings, which enable balanced 3-phase amplifiers to drive 6-phase coils.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of PPA Ser. Nr. 60/590,711, filedJul. 23, 2004.

FEDERALLY SPONSORED RESEARCH

None.

SEQUENCE LISTING

None.

BACKGROUND

This invention relates to linear motors forcers. Well known in the artare linear motor systems of the type known as Sawyer motors after theoriginal inventor Bruce Sawyer. These systems consist of a flat strip orsheet of magnetic material, such as electrical grade steel, withperiodic grooves cut or etched in the surface forming teeth, and anopposed mechanism with toothed electromagnetic elements. The steel stripor sheet is referred to as a platen, while the electromagnetic mechanismis referred to as a forcer.

If the Sawyer motor operates in only one direction, it is a linearmotor. The platen in this case is a strip with teeth cut in a directionorthogonal to the direction of motion. The forcer for a linear motor maycontain only a single set of electromagnets, although multiple sets maybe used to increase force. If the Sawyer motor operates in twodirections it is a planar motor. The platen in this case is a sheet withteeth cut in two orthogonal directions forming a grid of intersectinggrooves. The forcer for a planar motor must contain at least twoelectromagnets, one for each axis of motion, although moreelectromagnets are typically used. More specialized versions of bothlinear and planar Sawyer motors exist which use curved electromagnetsand curved or cylindrical platens. Such designs can provide linear aswell as rotational motion from a single motor.

The grooves formed in both the platen surface and the forcerelectromagnets, are typically filled with a strong, stable epoxy, andboth the platen surface and the toothed forcer surface are ground andlapped to provide an air-bearing quality surface. The forcer is providedwith a flexible cable assembly, termed an umbilical, which contains themotor coil leads and a compressed air supply tube. Passages in theforcer mechanism convey the compressed air supplied by the tube to anarraignment of small orifices or air jets, releasing the air into a thingap which forms between the forcer and the platen in opposition to thestrong magnetic attraction between these two members. Planar Sawyermotors almost universally utilize air-bearings. Linear Sawyer motors mayutilize air-bearings, ball or roller bearings, or recirculating ballslide units.

It is a characteristic of Sawyer motors that the electromagneticmechanism contains all permanent magnets and all coil windings, whilethe platen is fundamentally a passive magnetic element. Numerous otherlinear and planer motor designs exist wherein the platen may containpermanent magnets or coil windings. These are usually referred to asbrushless linear motors, moving coil motors, or AC linear motors.

This invention relates specifically to the design of hybrid variablereluctance electromagnets used in linear motor forcers and is notconcerned with the details of linear motor platens. A linear motor orplanar motor consists of the combination of both a forcer and a platen.The remainer of this disclosure will use the term forcer in referring tothe invention when not specifically discussing a forcer and a platen.

Numerous patents and technical publications have disclosed the prior artin this field. U.S. Pat. No. 3,376,578 to Sawyer described various3-phase variable reluctance magnetic structures for forcers. This patentalso disclosed the general equation for spacing single-phase elements toform a poly-phase forcer. U.S. Pat. No. 3,457,482 to Sawyer subsequentlydisclosed a hybrid variable reluctance design that incorporatedpermanent magnets. This design used two identical single-phaseelectromagnets to form a functional forcer. Next Bruce Sawyer disclosed(U.S. Pat. No. 3,836,835) an additional hybrid linear forcer designwherein one permanent magnet and two coils form a functional forcer, themost compact design to date.

Nocito, et al. (U.S. Pat. No. 3,878,411) disclosed an improved linearmotor design based on four of the previously disclosed single-phasehybrid variable reluctance elements. This design spaced the hybridvariable reluctance elements in an optimal manner to form a linear motorcapable of higher intrinsic positioning accuracy. This design is knownas the 2/4-phase motor design since although four single-phase elementsare used in each linear motor, the elements typically use coil windingscoupled in a particular manner to allow 2-phase drive.

These early patent disclosures typically show large U-shaped permanentmagnets coupling the back of two iron cores. This design was common whenlow-energy product ferrite or alnico magnets were used. With the adventof high-energy product rare earth magnets, a much smaller magneticvolume is required, and it is usually placed between the iron cores. Thetwo designs are sometimes referred to as exterior or back structure vs.interior or sandwich permanent magnet designs. The book Theory andApplication of Stem Motors (B. C. Kuo 1974) and Incremental MotionControl Step Motors and Control Systems (B. C. Kuo 1979) and the paperDirect Linear and Cartesian Coordinate Stepping Motors (Ed Pelta, 1985)provide a good technical description of these magnetic designs.

U.S. Pat. No. 6,533,035 to Smith disclosed a design wherein three of thewell known single-phase hybrid variable reluctance elements are spacedapart to form a 3-phase linear motor. U.S. Pat. No. 5,742,136 to Ono etal., disclosed a magnetically coupled 3-phase linear motor. JapanesePat. JP9261944 to Sutoki Riyuutarou disclosed a modified hybrid variablereluctance design wherein one permanent magnet and six coils from a3-phase linear motor.

To more fully understand the current invention a detailed description ofpertinent aspects of prior art is required. It should be noted that mostdescriptions of Sawyer motor operation approach the subject from anopen-loop perspective, assuming the motors are operated as open-loopsynchronous machines. As such, the drawings and discussion focus ontooth alignment and natural detent positions. In the discussion thatfollows the perspective is that of closed-loop control which focuses onforce production. Useful force is produced in electromagnet systems whenspace quadrature of the magnetic fields (M and H vectors) exists. Usefulmotion occurs when the magnetic fields translate or rotate with time.This leads to the well-known requirements of “space quadrature and timephase” for useful electromechanical energy conversion.

DESCRIPTION OF PRIOR ART—FIGS. 1, 2, 3, 4, 5A, 5B, 6, 7, 8

FIG. 1 shows a partial view of a magnetic system in ten differentstates. A magnetic core 13 is provided with one or more teeth 14 and isspaced apart by a small gap 12 from a second magnetic element 10provided with a plurality of teeth 11. Magnetic element 10 is termed aplaten. Core 13 is under the influence of an externally generatedmagnetomotive force (mmf) as shown by the arrow 15. Magnetic fluxproduced by mmf 15 descends through core 13, emerges from teeth 14,crosses gap 12 as denoted by wavy lines 16 and enters teeth 11 of platen10. Not shown is a necessary return magnetic path to complete thecircuit. In FIG. 1(A), core 13 is shown at five different relativepositions to platen 10. A direction of motion is indicated by arrow 17.Position is denoted in terms of phase where 360 degrees equates tomoving a distance equal to one tooth pitch.

In the position of 0 degrees, teeth 14 of core 13 are 180 degrees out ofalignment with teeth 11 of platen 10 resulting in high magneticreluctance and hence low magnetic flux. The flux bridges symmetricallyleft and right from teeth 14 to teeth 11. With core 13 displaced to 45degrees, a directional bias to the right is evident in the flux pathfrom teeth 14 to teeth 11. The flux density also increases due to lowerreluctance as the teeth become more aligned. The process continues asshown at 90 degrees and finally at 180 degrees where teeth 14 and teeth11 are fully aligned. At 180 degrees, the flux density is at a maximum,while reluctance is at a minimum. With a further displacement to 270degrees, it is seen that the flux exits teeth 14 with a bias to the leftwhile traversing gap 12 to teeth 11. Flux density is decreasing andreluctance is increasing for this position relative to the values at 180degrees.

If mmf 15 is held constant independent of position of core 13 relativeto platen 10, the varying reluctance of the toothed magnetic circuitwill result in varying magnetic energy stored in gap 12. The negativederivative of gap energy storage vs. displacement is the lateral forceproduced between core 13 and platen 10.

FIG. 2 shows force vs. displacement for two cases of mmf generation,constant and sinusoidal. For constant mmf excitation, force is asinusoidal function of position. Positive force in the +x direction isonly produced over the interval from 0 to 180 degrees. From 180 to 360degrees, force is produced in the −x direction. Peak force in +x occursat 90 degrees where teeth 14 and teeth 11 have 50% tooth overlapcorresponding to 90 degrees of misalignment. If useful force in the +xdirection is desired independent of position, clearly some means ofeliminating the negative force produced over the interval from 180 to360 degrees must be used.

At FIG. 1(B) the same five positions are shown with mmf 15 provided fromsome “intelligent” source linked to core 13 position. Specifically, letmmf 15 be given by the following equations:mmf=sin(P) for 0<=P<=180 degreesmmf=0 for 180<P<360 degrees.

Since sin(0)=0, there is no flux for the position of 0 degrees. Atposition 90 degrees, sin(90)=1 resulting in maximum flux. At position180 degrees, sin(180)=0 and flux is again 0. For all positions from 180to 360 degrees, flux is 0 since mmf 15 is zero for this region bydefinition of the controlling equations. FIG. 2 shows a curve for forceresulting from sinusoidal mmf. This curve produces positive +x force forthe region 0 to 180 degrees only, and no negative force for any region.The shape of this force curve in the region 0–180 degrees is that of asin² function and is always less than the sin curve of constant mmfexcept for the maximum at 90 degrees where the two curves are equal. Inpractice, the waveforms shown in FIG. 2 are not perfect functions ofsin, due to distortions arising from tooth geometry and magneticmaterials properties. Chapter 6 Permeance Model and Reluctance ForceBetween Toothed Structures by H. D. Chai in the book Theory andApplications of Step Motors (B. C. Kuo Ed. 1974) and the paper DirectLinear and Cartesian Coordinate Stepping Motors (Ed Pelta, 1985) provideadditional details on this subject.

The prior art disclosures for Sawyer motor designs are all based onconfiguring multiple toothed cores such as core 13 of FIG. 1 withsources of mmf in such a manner that continuos and smooth force ineither +x or −x direction may be obtained. It is additionally desired toprovide compact mechanisms with high force to weight ratios, highpositioning accuracy, good power efficiency while, and low manufacturingcost. As may be expected, it is difficult to achieve all of these goalsin a single design.

FIG. 3 shows a detailed view of a 2-phase Sawyer motor (U.S. Pat. No.3,457,482). A toothed platen 25 is spaced apart by an air gap 26 from apair of similar electromagnets 27 and 28. Electromagnet 27 is providedwith a first U-shaped magnetic core 30 spaced apart from a secondU-shaped magnetic core 32 by a rectangular permanent magnet 31. Core 30has a first toothed pole 33 spaced apart from a second toothed pole 34with a wire winding 37 disposed on core 30 between poles 33 and 34. Core32 has a first toothed pole 35 spaced apart from a second toothed pole36 with a wire winding 38 disposed on core 32 between poles 35 and 36.Electromagnet 28 is provided with a first U-shaped magnetic core 40spaced apart from a second U-shaped magnetic core 42 by a rectangularpermanent magnet 41. Core 40 has a first toothed pole 43 spaced apartfrom a second toothed pole 44 with a wire winding 47 disposed on core 40between poles 43 and 44. Core 42 has a first toothed pole 45 spacedapart from a second toothed pole 46 with a wire winding 48 disposed oncore 42 between poles 45 and 46. A gap 29 separates electromagnet 27from electromagnet 28.

The toothed poles 33, 34, 35, 36, 43, 44, 45, and 46 are spaced apartfrom each other in a precise manner relative to the teeth of platen 25with the teeth of each one of the respective poles having a relativephase offset. With the position shown being defined as an arbitraryzero, the relative phases may be assigned as follows:

Pole 33: offset 0 degrees, phase labeled a.

Pole 34: offset 180 degrees, phase labeled a′.

Pole 35: offset 0 degrees, phase labeled c.

Pole 36: offset 180 degrees, phase labeled c′.

Pole 43: offset 90 degrees, phase labeled b.

Pole 44: offset 270 degrees, phase labeled b′.

Pole 45: offset 90 degrees, phase labeled d.

Pole 46: offset 270 degrees, phase labeled d′.

Each of the poles 33, 34, 35, 36, 43, 44, 45, and 46 of FIG. 3 are seento be similar to core 13 of FIG. 1. If some means of inducing asinusoidal mmf in these cores can be effected, each will produce a sin²type force function over a 180-degree range as was seen in FIG. 2. Forthe position shown, with the intent of inducing motion in the+x-direction the function of electromagnet 28 will be considered first.

Permanent magnet 41 provides a mmf, which biases core 42 to a magnetic Npotential and biases core 40 to a magnetic S potential. If this were theonly source of mmf in electromagnet 28, flux would distribute in poles43, 44, 45, and 46 according to the reluctance in air gap 26 betweentoothed poles 43, 44, 45, and 46 and toothed-platen 25. If a constantmmf were applied to pole 46 a sinusoidal force vs. position functionwould result as shown in FIG. 2. If a constant mmf were applied to pole45 a sinusoidal force vs. position function would result as well.However, the sinusoidal force function for pole 46 would be 180 degreesout of phase relative to the sinusoidal force function for pole 45 thusexactly canceling each other out. In the same way, pole 43 is displaced180 degrees from pole 44, hence the combined pole pairs under constantmmf cannot produce any net force.

The addition of winding 48 on core 42 and winding 47 on core 40 providesa means of controllable altering the mmf seen by each of the poles 43,44, 45, and 46. If current is caused to flow into the +terminal ofwinding 48, then a mmf vector 50 is produced which decreases flux inpole 46 but increases flux in pole 45. Similiarly, if current is causedto flow into the + terminal of winding 47, then a mmf vector 49 isproduced which decreases flux in pole 44 but increases flux in pole 43.Windings 47 and 48 are actually in phase, and may be connected in eitherseries or parallel and treated as a single phase. In fact, windings 47and 48 are often formed using a single winding disposed in the slotbetween poles 43 and 44 to the slot between poles 45 and 46. If windings47 and 48 are driven by a current which is sinusoidal with respect tothe position of electromagnet 28 relative to platen 25, then the mmfseen by poles 43, 44, 45, and 46 is similar to the case shown in FIG.1(B) for sinusoidal mmf excitation of a single core. Poles 33, 34, 35,and 36 and windings 37 and 38 of electromagnet 27 function in a similarmanner.

FIG. 4 shows the net force produced by electromagnets 27 and 28 assumingappropriately phased sinusoidal currents are commanded to windings 37,38, 47, and 48 based on the relative phase position of electromagnets 27and 28 to platen 25. For the starting position shown, and for motion inthe +x-direction as shown by an arrow 24, windings 47 and 48 wouldrequire a cosine current function (cos(0)=1) while windings 37 and 38would require a sine function (sin(0)=0). Poles 43 and 45 are in phaseand produce identical force functions. Similarly, poles 44 and 46 arepaired, poles 33 and 35 are paired, and poles 34 and 36 are paired. Eachin phase pole pair produces a phased sin² type force function over some180-degree interval, and is 0 for the remaining 180 degrees of each360-degree tooth pitch. The sum of two such functions that are 180degrees apart is in fact some phased sin² over the entire 360 degrees.Thus the force function of poles 43 and 45 summed with the forcefunction of poles 44 and 46 is cos², while the force function of poles33 and 35 summed with the force function of poles 34 and 36 is sin².Since sin²+cos²=1, it can be seen that the design shown in FIG. 3 iscapable of producing constant force independent of position. It shouldbe noted that a more detailed examination of force produced by each polewould show some negative and positive forces produced during the180-degree interval that is assumed to be zero force in FIG. 4. Theseadditional forces when summed over all the poles of each electromagnetcancel out reasonable well, leaving the net forces shown in FIG. 4.

The design shown in FIG. 3 is the now classic 2-phase Sawyer motordesign. Some important aspects of this design will be qualitativelynoted. This design is one example of a class of electrical machinesreferred to as homopolar synchronous machines. Further it is a doublesalient permanent magnet machine (DSPM). Referring to electromagnet 28,core 40 has two discrete or salient poles, 43 and 44, while core 42 hastwo discrete or salient poles, 45 and 46. Permanent magnet 41 alwaysbiases core 42 and salient poles 45 and 46 to N potential while core 40and salient poles 43 and 44 are biased to S potential. Henceelectromagnet 28 is homopolar, that is the magnetic field neverreverses. The function of winding 47 is to differentially steer fluxthrough salient poles 43 and 44 while winding 48 provides the samefunction with respect to salient poles 45 and 46. Flux always emergesfrom poles 45 and 46, crosses gap 26, flows along platen 25, crosses gap26 and enters poles 43 and 44. The mmf produced in windings 47 and 48 isrestricted to a range that never reverses this flux flow. The machine issynchronous in that force production depends on winding current beingcontrolled as a function of the position of electromagnets 27 and 28relative to platen 25. This control may be open-loop or closed-loopdepending on whether some means of actually sensing the position isused.

Electromagnets 27 and 28 are each considered single-phase elementsdespite having four salient poles each. As previously noted, in phasepole pairs produce in phase force functions, hence each electromagnethas only two distinct magnet poles. Further, each pole pair can onlyattract to platen 25, there is no mechanism for repulsion in thisdesign. This is a consequence of platen 25 being magnetically passive asis typical of strictly variable reluctance elements. Any pole that is+/−180 degrees out of phase to another pole is part of the sameelectrical phase.

The design of FIG. 3 shows permanent magnet 31 placed between cores 30and 32, and permanent magnet 41 placed between cores 40 and 42. This isthe so-called interior or sandwich magnetic design. Rare earth magnetssuch as samarium-cobalt are used for this purpose. Since thepermeability of rare earth magnets is close to unity, the magnetfunctions as a large air gap in the circuit in addition to providing abias mmf. This isolates the mmf effects of coil 38 from coil 37 and coil48 from coil 47. Examining electromagnet 28, the flux through magnet 41remains constant at all times independent of the position ofelectromagnet 28 relative to platen 25 or the operating conditions ofwindings 47 and 48. The net flux flowing through platen 25 directlyadjacent to magnet 41 is also constant. However, the position ofelectromagnet 28 relative to platen 25 in conjunction with mmf inducedby windings 47 and 48 causes variation in the flux path of the magneticcircuit. For the state shown in FIG. 3, flux is shown circulating downpole 45, through platen 25, returning up pole 43 and across magnet 41.When electromagnet 28 shifts 180 degrees in the +x-direction, flux willcirculate down pole 46, through platen 25, up pole 44, and across magnet41. It can thus be seen that the magnet flux path effectively oscillatesback and forth relative to the central location of magnet 41 inelectromagnet 28 depending on instantaneous operating conditions.

A very significant aspect of this single-phase electromagnet designshould be noted. Pole 43 of core 40 is further from magnet 41 than pole44. It is in phase with pole 45 of core 42 that is closer to magnet 41than pole 46. Thus it is seen that in phase pole pair 43 and 45 has botha close and more distant location to magnet 41, while the same is alsotrue of in phase pole pair 44 and 46. There are three consequences ofthis arrangement. First, the effective flux path length staysapproximately constant independent of operating condition, however itdoes oscillate back and forth spatially as noted above. Secondly, poles44 and 45 which are adjacent to magnet 41 but of opposite magneticpolarity are 180 degrees out of phase. This allows windings 47 and 48 tobe formed as a single winding from the slot between poles 43 and 44 tothe slot between poles 45 and 46. Such a winding encloses poles 44 and45 and part of magnet 41. The third consequence of this arrangement isimproved linearity in the magnetic circuit. Since pole 44 is closer tomagnet 41 than pole 43, magnetic reluctance is less for pole 44 thanpole 43. However, the reverse is true for the respective phase poles 46and 45 on the opposite side of magnet 41. Thus each in phase pole pairsuch as 43 and 45 has one higher reluctance member such as 43 and onelower reluctance member such as 45 in the magnetic path reluctance.Designs, which do not balance magnetic path length and magnetic pathreluctance, are to be avoided.

It should be further noted that the orientation of permanent magnets 31and 41 are in opposition to each other. The purpose of alternatingorientation is to effectively isolate the magnetic circuit ofelectromagnet 27 from the magnetic circuit of electromagnet 28. Althoughthis was disclosed in the drawings of Bruce Sawyer's patents, a bettertechnical description can be found in the paper Linear Stepping Motorwith Uncoupled Phases by J. P. Pawletko and H. D. Chai (1984). Thispaper describes a 3-phase variable reluctance design, but the principleis identical to reverse biased permanent magnet designs.

As previously noted, an alternate position of permanent magnets known asexterior, outer, or back-surface designs also exists. Many windingvariations have also been disclosed. The original patent to Sawyer (U.S.Pat. No. 3,457,482) illustrates some of these alternatives.

FIG. 5A illustrates an improvement (U.S. Pat. No. 3,836,835) based onthe single-phase electromagnet design of FIG. 3. A platen 60 is spacedapart by an air gap 61 from an electromagnet 62. Electromagnet 62consists of a first magnetic core 63 having a pair of toothed poles 66and 67 spaced apart by a permanent magnet 64 from a second magnetic core65 having a pair of toothed poles 68 and 69. A winding 70 is providedbetween poles 66 and 67 of core 63 while a winding 71 is providedbetween poles 68 and 69 of core 65. The elements of electromagnet 62 ofFIG. 5A are mechanically and functionally similar to the elements ofelectromagnets 27 or 28 of FIG. 3. The one important difference is therelative phase spacing of poles 66, 67, 68, and 69. If pole 66 isdefined as being at 0 degrees, then pole 67 is at 180 degrees, whilepole 68 is at 90 degrees and pole 69 is at 270 degrees. Thus it can beseen that poles 66 and 67 on one side of magnet 64 have a relationshipof 180 degrees to each other, and similarly poles 68 and 69 on theopposite side of magnet 64 also have a relationship of 180 degrees toeach other. This is as the conditions in FIG. 3 except for the fact thatmatching phase pole pairs across magnet 64 do not exist.

Magnet 64 provides a net bias mmf between cores 63 and 65 while windings70 and 71 provide steering mmf to controllable direct flux through thedesired poles 66, 67, 68 and 69 depending on the position ofelectromagnet 62 relative to platen 60 and the desired direction offorce generation. FIG. 5A shows the operating condition for maximumforce generation in the +x-direction, as shown by an arrow 73, with pole68 being in the 90-degree misalignment state generating maximum force.For this state, winding 71 is energized (cos(0)=1) to produce a mmfvector 72 which increases flux in pole 68 while canceling flux in pole69. At the same instant winding 70 is not energized (sin(0)=0) and fluxis free to find the path of least reluctance through either pole 66 or67 which in this instance is pole 67.

A plot of net force produced from the design shown in FIG. 5A would besimilar to the plot of FIG. 4, but would have one half the amplitude.This is a result of electromagnet 62 of FIG. 5A having one-half theactive pole area of combined electromagnets 27 and 28 of FIG. 3. Thesignificance of this design is that a single electromagnet such as 62can produce position independent force (i.e. it is useful motor) whenequipped with two windings and four poles having the phase relationshipdescribed above. For the same geometrical size of electromagnets thisenables a motor approximately one-half the size of that obtainable withthe design of FIG. 3 with corresponding one-half force.

FIG. 5B shows the design of FIG. 5A in the same relative position butwith winding 71 energized to produce a mmf vector 74 for generatingforce in the opposite direction as shown by an arrow 75. Flux isconcentrated through pole 69, which is the force-producing pole, andagain returns via aligned pole 67. A comparison of flux paths betweenFIG. 5A and FIG. 5B illustrates that operating condition changes resultin variable magnetic flux path lengths. In addition, poles 66 and 69 arefurther from magnet 64 than poles 67 and 68. Since each of poles 66, 67,68 and 69 have unique phase, there are nonsymmetrical magnetic pathsfrom magnet 64 to each individual phase. As a result, the single elementtwo-phase electromagnet design of FIGS. 5A and 5B is less linear thanthe two-element single-phase electromagnet design of FIG. 3. Finally itshould be noted that if the electromagnets are comparably sized, and thedesired motor force equal, than two similar elements as shown in FIG. 5must be used in parallel to obtain the same force as the design of FIG.3, hence there is no cost advantage either.

FIG. 6 illustrates the next major improvement in Sawyer motor design asdisclosed by Nocito, et al. in U.S. Pat. No. 3,878,411. In thediscussion of FIG. 2 and FIG. 4 above, pole force functions were statedto be of a sin type. In reality, it is a distorted sin² function withthe major distortion being the fourth order harmonic of tooth pitch.This can be conceptually understood by noting that FIG. 4 contains fourpeaks. If these individual functions are distorted, their sum is nolonger a constant but will contain a 4'th order distortion. Another wayto understand this is that sin² t=½(sin 2t+1) so that the 1'st harmonicof the force function is the 4'th harmonic of the tooth pitch. Thisforce function distortion effects positioning accuracy in open-loopsystems and produces non-linear force ripple in close-loop systems. Inopen-loop systems it can lead to significant resonant behavior whenmotors slew at velocities where the force distortion frequency matchesthe mechanical resonant frequency of the motor. Even in closed-loopsystems, suppression of this resonant behavior may be an issue.

The solution proposed by Nocito was to double the number of single-phaseelectromagnets and space them in a manner that caused the 4'th orderdistortion to cancel. FIG. 6 schematically shows four single-phaseelectromagnets of the type shown in FIG. 3. A first electromagnet 80provides poles at 0 and 180 degrees. A second electromagnet 81 is spacedapart to provide poles at 90 and 270 degrees. A third electromagnet 83is spaced apart to provide poles at 45 and 225 degrees. A fourthelectromagnet 83 is spaced apart to provide poles at 135 and 315degrees.

Electromagnet 80 is provided with a single-phase coil 84 that isconnected to phase a. Electromagnet 81 is provided with a single-phasecoil 85 that is connected to phase b. Electromagnet 82 is provided witha split-phase coil that is derived from both phase a and phase b asshown. Electromagnet 83 is provided with a split-phase coil that isderived from both phase a and phase b as shown. A plot of pole forcefunctions for the design of FIG. 6 would contain eight peaks spaced 45degrees apart rather than four peaks spaced 90 degrees apart as for thedesign of FIG. 3. Unfortunately the design of FIG. 6 doubles the numberof electromagnets needed to build a motor, increasing motor size andcost.

Although Nocito did not disclose it, the same approach can be applied tothe single-element, two-phase electromagnet of FIGS. 5A and 5B. FIG. 7shows a schematic representation of such a design. A first electromagnet90 is equipped with poles at 0, 180, 90 and 270 degrees while a secondsimilar electromagnet 91 is spaced apart to provide poles at 45, 225,135 and 315 degrees. Electromagnet 90 is provided with two single-phasewindings 92 and 93 connected to phase a and phase b respectively.Electromagnet 91 is provided with two split-phase windings 94 and 95both of which are connected to both phase a and phase b.

Both of the designs shown in FIG. 6 and FIG. 7 are of the type known as2/4-phase motors. They are driven as two-phase systems due to thecoupled windings on the second set of electromagnets. They produce forceimpulses according to a true balanced 4-phase configuration.Magnetically the design of FIG. 6 is more linear as previously noted,whereas the design of FIG. 8 allows more compact motors. A disadvantageof both designs shown in FIG. 6 and FIG. 7 are that two differentelectromagnet winding configurations are required for constructing amotor. This complicates manufacturing somewhat and increasesmanufacturing cost.

FIG. 8 shows a schematic representation of a 3-phase design disclosed bySmit in U.S. Pat. No. 6,533,035, wherein three of the single-phaseelements of FIG. 3 are spaced apart in the well-known 0, 120, 240 degreebalanced offsets of 3-phase systems. A first electromagnet 100 isprovided with poles spaced at 0 and 180 degrees. A second electromagnet101 is spaced apart to provide poles at 120 and 300 degrees. A thirdelectromagnet 102 is spaced apart to provide poles at 240 and 60degrees. Electromagnet 100 is provided with a coil 103 that isdesignated phase a. Electromagnet 101 is provided with a coil 104 thatis designated phase b. Electromagnet 102 is provided with a coil 105that is designated phase c. It is customary to further connect the leadsof phases a, b, and c in either a delta or Y configuration for drivingthe coils from a 3-phase power amplifier. The design of FIG. 8 producesa force function with six peaks, falling midway between the four-peakforce function of the designs of FIG. 3 and FIG. 5A and the eight peakforce functions of the designs of FIG. 6 and FIG. 7. It is alsointermediate in size requiring three elements.

The 2/4-phase designs are typically driven from dual H-bridge poweramplifiers, which is equivalent to four T-bridge power amplifiers. Adelta or Y connected 3-phase design is driven from a triple T-bridgepower amplifier. The 3-phase design is therefor more economical to drivethan the 2/4-phase designs.

BACKGROUND OF INVENTION—OBJECTS AND ADVANTAGES

Accordingly, several objects and advantages of the present inventionare:

-   -   (a) to provide an improved Sawyer motor design which improves        the linearity of pole force function beyond that obtained with        the eight pulse 2/4-phase design;    -   (b) to provide an improved Sawyer motor design which requires        fewer electromagnets than the four single-phase electromagnets        used in the 2/4-phase design;    -   (c) to provide an improved Sawyer motor design which can be        efficiently driven from 3-phase power amplifiers as either a        delta or Y connected machine;    -   (d) to provide an improved Sawyer motor design wherein all        electromagnets and there associated windings are identical, thus        reducing manufacturing costs.

Additional objects and advantages of the invention will become apparentfrom the following description and drawings.

SUMMARY

The above objects are accomplished by utilizing a new 2-phaseelectromagnet element with three identical such elements coupled in amanner to provide 6-phase force production while being compatible with3-phase amplifiers. The pole force function has 12 peaks spaced 30degrees apart which sum to provide smooth continues force independent ofposition. A split-phase winding allows the six motor phases to be drivenfrom a standard 3-phase amplifier and also results in identical windingsfor all three electromagnets. The resulting system has lower forceripple than any existing design, is more compact than the four element2/4 phase design and is cheaper to drive.

DRAWINGS—FIGURES

Sheet 1:

FIG. 1: Single pole magnetic flux behavior of prior art system.

FIG. 2: Force vs. position curves for single pole prior art system.

Sheet 2:

FIG. 3: Dual electromagnet 2-phase prior art system.

FIG. 4: Force vs. position curves for 2-phase prior art system.

Sheet 3:

FIG. 5A: Single electromagnet 2-phase prior art system.

FIG. 5B: Single electromagnet 2-phase prior art system in reverse force.

Sheet 4:

FIG. 6: Schematic diagram of quad electromagnet 2/4-phase prior artsystem.

FIG. 7: Schematic diagram of dual electromagnet 2/4-phase prior artsystem.

Sheet 5:

FIG. 8: Schematic diagram of triple electromagnet 3-phase prior artsystem.

FIG. 9: Schematic diagram of triple electromagnet 3/6-phase system.

Sheet 6:

FIG. 10: Perspective view of triple electromagnet 3/6-phase system.

FIG. 11: Force vs. position curves for a single electromagnet of the3/6-phase system.

Sheet 7:

FIG. 12: Triple electromagnet 3/6-phase system.

Sheet 8:

FIG. 13: Phase diagram of 3/6-phase system.

FIG. 14: Phase diagram of 2/6-phase system.

Sheet 9:

FIG. 15: Alternate winding configuration of electromagnet.

FIG. 16: Alternate electromagnet design.

Sheet 10:

FIG. 17A: Front perspective view of planar Sawyer motor with 3/6-phaseelectromagnets.

FIG. 17B: Rear perspective view of planar Sawyer motor with 3/6-phaseelectromagnets.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT—FIGS. 9, 10, 12

FIG. 10 shows a magnetic platen 201 provided with a pattern of grooves202 forming a sequence of teeth 203 having a constant pitch 204. Thegrooves 202 and the teeth 203 are of approximately equal width. A set ofthree electromagnets 206, 208 and 210 are disposed apart from platen 201by a small air gap 205. A direction of travel is indicated by arrow 241.

Electromagnet 206 has a first U-shaped magnetic core 211 spaced apart bya permanent magnet 212 from a second similar U-shaped magnetic core 213.Core 211 has a first salient toothed pole 216 spaced apart from a secondsalient toothed pole 217. A winding 214 is provided in the slot betweensalient pole 216 and salient pole 217. Core 213 has a first salienttoothed pole 218 spaced apart from a second salient toothed pole 219. Awinding 215 is provided in the slot between salient pole 218 and salientpole 219.

Electromagnet 208 has a first U-shaped magnetic core 221 spaced apart bya permanent magnet 222 from a second similar U-shaped magnetic core 223.Core 221 has a first salient toothed pole 226 spaced apart from a secondsalient toothed pole 227. A winding 224 is provided in the slot betweensalient pole 226 and salient pole 227. Core 223 has a first salienttoothed pole 228 spaced apart from a second salient toothed pole 229. Awinding 225 is provided in the slot between salient pole 228 and salientpole 229.

Electromagnet 210 has a first U-shaped magnetic core 231 spaced apart bya permanent magnet 232 from a second similar U-shaped magnetic core 233.Core 231 has a first salient toothed pole 236 spaced apart from a secondsalient toothed pole 237. A winding 234 is provided in the slot betweensalient pole 236 and salient pole 237. Core 233 has a first salienttoothed pole 238 spaced apart from a second salient toothed pole 239. Awinding 235 is provided in the slot between salient pole 238 and salientpole 239. The tooth pitch of salient poles 216, 217, 218, 219, 226, 227,228, 229, 236, 237, 238 and 239 is the same as tooth pitch 204 of platen201, and the tooth geometry is similar to that of platen 201.

Electromagnet 206 is spaced apart by a gap 207 from electromagnet 208.Electromagnet 208 is spaced apart by a gap 209 from electromagnet 210.Not shown is some necessary mechanical structure for rigidly mountingelectromagnets 206, 208 and 210 to precisely maintain gaps 207 and 209and to provide some means of maintaining gap 205. Such structures aretypically machined or cast aluminum housings with pockets provided forembedding electromagnets 206, 208, and 210 using an epoxy pottingcompound. The housing typically provides a bearing system such as an airbearing for maintaining gap 205 to allow relative motion ofelectromagnets 206, 208 and 210 to platen 201 in the direction shown byarrow 241.

FIG. 12 provides a more detailed 2D view of the system of FIG. 10 usingthe same numbers for similar parts. Windings 214, 215, 224, 225, 234,and 235 are shown as split phase windings with four leads, a detail notshown in FIG. 10. Thus winding 214 consists of a pair of two discretewindings, 214 a and 214 b. Winding 215 consists of a pair of twodiscrete windings 215 a and 215 b. Winding 224 consists of a pair of twodiscrete windings 224 a and 224 b. Winding 225 consists of a pair of twodiscrete windings 225 a and 225 b. Winding 234 consists of a pair of twodiscrete windings 234 a and 234 b. Winding 235 consists of a pair of twodiscrete windings 235 a and 235 b.

The orientation of permanent magnets 212, 222, and 232 is also shown inFIG. 12 as designated by N and S marks. Permanent magnets 212 and 232have the same orientation, while permanent magnet 222 has reversedorientation.

The relative phase of each salient pole is also more clearly visible andmay be assigned a label as follows:

Electromagnet 206:

Salient pole 216: 0 degrees/phase a′

Salient pole 217: 180 degrees/phase a′

Salient pole 218: 30 degrees/phase d

Salient pole 219: 210 degrees/phase d′

Electromagnet 208:

Salient pole 226: 120 degrees/phase b

Salient pole 227: 300 degrees/phase b′

Salient pole 228: 150 degrees/phase e

Salient pole 229: 330 degrees/phase e′

Electromagnet 210:

Salient pole 236: 240 degrees/phase c

Salient pole 237: 60 degrees/phase c′

Salient pole 238: 270 degrees/phase f

Salient pole 239: 90 degrees/phase f′

The twelve individual toothed salient poles are thus seen to each have aunique phase spacing at some multiple of 30° relative to the otherpoles. A general equation may be written as: P=+/−N×30°, where N=0, 1,2, 3, 4, 5, 6, 7, 8, 9, 10, or 11. However, the two salient poles on anygiven magnetic core must have a phase relationship of +/−180° relativeto each other, that is they must be part of a phase pair. The actualordering of the three electromagnets need not be as stated in the listabove.

FIG. 9 shows a schematic diagram of electromagnets 206, 208, and 210showing each salient pole and coil windings. A balanced three phasecurrent source, not shown, provides current phases i, j, and k. Windings214 a, 215 a, 225 b, and 234 b are connected to phase i. Windings 214 b,224 a, 225 a, and 235 b are connected to phase j. Windings 215 b, 224 b,234 a, and 235 a are connected to phase k. FIG. 9 shows all windings ofa given phase as series connected which ensures equal current flow. Itis also possible to parallel connect the windings of a given phase,although equal current distribution is then dependent on the magneticproperties of each electromagnet assembly. It should be noted that thecorrect phase polarity must be determined when connecting each winding.This depends on permanent magnet orientation and handedness of coilwindings, both details not shown in FIG. 9. Reversing lead connectionsof a phase inverts the phase.

Operation of Preferred Embodiment—FIGS. 9, 11, 12, 13

FIG. 12 shows electromagnets 206, 208 and 210 in what may be defined asa zero offset or reference position relative to platen 201. Assume it isdesired to produce force in the +x direction as indicated by arrow 241.Peak force for this condition will be developed by whichever salientpole lags platen 201 by 90 degrees. This is found to be salient pole 238designated phase f. Given the orientation shown for permanent magnet232, magnetic core 233 is biased at S magnetic potential while magneticcore 231 is biased to N magnetic potential. A steering mmf must beproduced by the sum of windings 235 a and 235 b as shown by the heavyarrow in order to concentrate flux in core 233 to salient pole 238 whileblocking flux in salient pole 239. The return flux path through core 231is primarily through salient pole 236 designated phase c, which lagsplaten 201 by 120 degrees for the position shown. A suitable steeringmmf must be produced by the sum of windings 234 a and 234 b as shown bya lighter arrow.

If windings 235 a and 235 b sum to produce a mmf which is a function ofcos(p), and windings 234 a and 234 b sum to produce a mmf which is afunction of cos(p−30) where p is position phase in degrees, then netforce vs. displacement curves for phases c, c′, f and f′ of salientpoles 236, 237, 238 and 239 will be as shown in FIG. 11. As alreadydescribed for FIGS. 1–4, each salient pole produces a sin² forcefunction over a 180 degree interval and is zero for the remaining 180degrees of each 360 degree tooth pitch. The sum of two salient poles 180degrees out of phase contained on a single magnetic core such as 233 isa phased sin² force function over the full 360 degrees. Salient poles236 and 237 form a first pair of 180-degree poles while poles 238 and239 form a second pair of 180-degree poles. The relative spacing ofpoles 236 and 237 to poles 238 and 239 is 30 degrees. Thus the forcefunction shown in FIG. 11 is that of cos²(p) and cos²(p−30). The totalforce function of electromagnets 206, 208 and 210 combined has threesimilar graphs as shown in FIG. 11 having 12 peaks summing to a constantaccording to the following equation:F _(total)=k[cos²(p)+cos²(p−30)+cos²(p−60)+cos²(p−90)+cos²(p−120)+cos²(p−150)]=3k

Where p=position phase in degrees and k is a constant for the forceproduced from each magnetic core 211, 213, 221, 223, 231, and 233. Thetotal force is thus independent of position p.

FIG. 13 shows a vector diagram of the phases depicted in FIG. 12. Phasea is chosen as 0. Phases a, d, c′, f′, b, e, a′, d′, c, f, b′, and e′are equally spaced 30 degrees apart. Each letter and its prime such as fand f′ are seen to be 180 degrees apart. Each pair of 180 degree phasesis more accurately considered a single phase and is always formed by twosalient poles such as 238 and 239 on a single magnetic core such as 233of FIG. 12. The system of FIGS. 10 and 12 is thus considered a true6-phase system and the force function will consist of 12 pulses equallyspaced 30 degrees apart. FIG. 13 shows the non-prime phase designationsa, b, c, d, e, and f as being the actual six phases using heavy arrows.The primed phases a′, b′, c′, d′, e′, and f′ are shown using lightarrows. FIG. 13 more clearly shows which phases of FIG. 12 produce forceas needed for the position shown in FIG. 12 with phase a=0. For force inthe +x direction of arrow 241 of FIG. 12 or counter clockwise rotationin FIG. 13, all phases which lag phase a between 0 and 180 degrees willproduce useful force. Thus phases e′, b′, f, c, and d′ will all producepositive force with f which lags 90 degrees producing peak force. Forforce in the −x direction of arrow 241 of FIG. 12 or clockwise rotationin FIG. 13, all phases which lag phase a between 0 and 180 degrees willproduce useful force. Thus phases d, c′, f′, b, and e will all produceforce with f′ which lags by 90 degrees producing peak force. The vectorsthat lag the current zero position depends on the desired direction oftravel.

Referring to FIG. 12 and considering electromagnet 210 it can be seenthat magnetic path length behavior is more similar to single-phaseelement 28 of FIG. 3 than to two-phase element 62 of FIG. 5A. When fluxis concentrated through pole 238 which is adjacent to magnet 232 itprimarily returns through pole 236 which is more distant to magnet 232.Similarly, when flux is concentrated through pole 239 which is moredistant to magnet 232 it will primarily return through pole 237 which isadjacent to magnet 232. The relative phase offset of core 42 from core40 in FIG. 3 is 0 degrees which gives the best magnetic path lengthstability. The relative offset of core 233 to core 234 in FIG. 12 is 30degrees, which gives good magnetic path length stability. The relativephase offset of core 65 to core 63 in FIG. 5A is 90 degrees which givesthe poorest magnetic path length stability.

These are many other combinations of pole spacing that produce working6-phase designs, but the preferred spacing is that shown in FIG. 9 andFIG. 12 or the following equivalent series which just reverses theleft/right lead/lag relationship:

Electromagnet 206:

Salient pole 216: 0 degrees/phase a

Salient pole 217: 180 degrees/phase a′

Salient pole 218: 330 degrees/phase e′

Salient pole 219: 150 degrees/phase e

Electromagnet 208:

Salient pole 226: 120 degrees/phase b

Salient pole 227: 300 degrees/phase b′

Salient pole 228: 90 degrees/phase f′

Salient pole 229: 270 degrees/phase f

Electromagnet 210:

Salient pole 236: 240 degrees/phase c

Salient pole 237: 60 degrees/phase c′

Salient pole 238: 210 degrees/phase d′

Salient pole 239: 30 degrees/phase d

The preferred phase spacing places a first 180° pole pair with a second180° pole pair on each electromagnet where the first pole pair is offset+/−30° from the second pole pair. However, other combinations are alsouseful, such as a first pole pair offset +/−60° or +/−90°, from a secondpole pair of each electromagnet. In addition, the identicalelectromagnets such as 206, 208, and 210 may be space apart from eachother by phases such as 300 or 600 rather than 120°. An example of aphase configuration using 30° core to core spacing within theelectromagnets with 60° electromagnet to electromagnet spacing is:

Electromagnet 206:

Salient pole 216: 0 degrees/phase a

Salient pole 217: 180 degrees/phase a′

Salient pole 218: 30 degrees/phase d

Salient pole 219: 210 degrees/phase d′

Electromagnet 208:

Salient pole 226: 60 degrees/phase b

Salient pole 227: 240 degrees/phase b′

Salient pole 228: 90 degrees/phase e

Salient pole 229: 270 degrees/phase e′

Electromagnet 210:

Salient pole 236: 120 degrees/phase c

Salient pole 237: 300 degrees/phase c′

Salient pole 238: 150 degrees/phase f

Salient pole 239: 330 degrees/phase f′

Another possible phase configuration using 90° core to core spacingwithin the electromagnets with 30° electromagnet to electromagnetspacing is:

Electromagnet 206:

Salient pole 216: 0 degrees/phase a

Salient pole 217: 180 degrees/phase a′

Salient pole 218: 90 degrees/phase d

Salient pole 219: 270 degrees/phase d′

Electromagnet 208:

Salient pole 226: 30 degrees/phase b

Salient pole 227: 210 degrees/phase b′

Salient pole 228: 120 degrees/phase e

Salient pole 229: 300 degrees/phase e′

Electromagnet 210:

Salient pole 236: 60 degrees/phase c

Salient pole 237: 240 degrees/phase c′

Salient pole 238: 150 degrees/phase f

Salient pole 239: 330 degrees/phase f′

One skilled in the art will realize that a given choice of electromagnetcore to core phase as well as the choice of electromagnet toelectromagnet phase will impact the overall minimum size of the forcer,since the discrete nature of the teeth impose geometrical constraints.These overall size variations are most pronounced if each pole has asingle large tooth compared to poles with 4 to 6 fine teeth fine. Therequired thickness of permanent magnet as well as any mechanicalmounting requirements must also be considered when selecting the actualspacing between cores within electromagnets as well as the spacingbetween electromagnets. The correct vector diagram for each specificphase configuration must be determined to correctly wind andinterconnect the six individual split-phase windings.

While the above phase selections result in mechanically and electricallyidentical electromagnets, other variations can be specified whichrequire different core to core phase spacing resulting in two or threedifferent mechanical and electrical electromagnets in a forcer. Sincethis generally increases manufacturing cost as well as decreases systemlinearity, such designs should generally be avoided.

One skilled in the art will realize that a +/−120° core to core phase ona single electromagnet reduces the 6-phase design to a 3-phase design,where each electromagnet is a coupled 2-phase pair. One such phaseconfiguration is:

Electromagnet 206:

Salient pole 216: 0 degrees/phase a

Salient pole 217: 180 degrees/phase a′

Salient pole 218: 120 degrees/phase b

Salient pole 219: 300 degrees/phase b′

Electromagnet 208:

Salient pole 226: 120 degrees/phase b

Salient pole 227: 300 degrees/phase b′

Salient pole 228: 240 degrees/phase c

Salient pole 229: 60 degrees/phase c′

Electromagnet 210:

Salient pole 236: 240 degrees/phase c

Salient pole 237: 60 degrees/phase c′

Salient pole 238: 0 degrees/phase a

Salient pole 239: 180 degrees/phase a′

Each phase winding of this configuration is a pure single-phase witheach phase being distributed across two electromagnets. The motor energyefficiency is identical to a standard three-phase motor, while thelinearity may be slightly improved due to the spatial distribution offorce production compared to a standard three-phase motor. Thisconfiguration produces a 6-pulse force function similar to a 3-phasedesign.

FIG. 13 shows balanced three-phase vectors i, j, and k, offset 15degrees from phase a to provide a means of generating a six-phase driveusing standard 3-phase drive electronics. A vector 250 labeled mi isshown in the direction of i, while a second vector 251 labeled −nk isshown attached to vector 250's tip. Vectors 250 and 251 sum to producephase d. Note that for drawing clarity, vectors 250 and 251 are shownoffset from the origin and offset from phase d. Similar vectors may bechosen to construct all six phases as needed. The following equationsprovide the correct split phase winding turns required for theconfiguration shown in FIGS. 9, 12, and 13.a=mi−njd=mi−nkb=mj−nke=mj−nic=mk−nif=mk−nj

where m=0.8165 and n=0.2988 are scale factors applied to the totalnumber of turns used for a 6-phase design.

An important aspect of the above equations is that each winding hasidentical m and n fractions of two primary phases. Thus all windings areidentical for manufacturing purposes, reducing cost. To one skilled inthe art, it is apparent that there are many possible windingcombinations, which can produce the same net mmf starting from differentphase relationships.

Since m+n=1.115 it can be seen that 11.5% more turns are required toachieve an equal number of ampere-turns for a three-phase drive asopposed to a six-phase drive. If the same number of total turns is used,split according to the ratio of m and n as given, then 11.5% morecurrent must be supplied to generate the same motor force while theeffective winding resistance would be 0.8969 of nominal. This results ina motor constant (force output/square root of watts input) of 0.897 or11.5% worse than a six-phase drive. This is the penalty paid forachieving three-phase drive of a six-phase motor. This penalty may besomewhat offset by improving winding fill factors through the use offlat ribbon, square or hexagonal magnet wire. Power dissipation improveslinearly with fill factor while motor constant improves by the squareroot of fill factor. The decreased motor efficiency is offset however byincreased overall amplifier efficiency due to half the number ofamplifiers being required for 3-phase drive vs. 6-phase drive.

Additional Embodiments—FIG. 17A, 17B

FIGS. 17A and 17B depict an additional embodiment of the presentinvention in which four 3/6-phase linear motor forcers such as is shownin FIG. 10 and FIG. 12 are combined to form a two-axis planar motorforcer 300. Referring to FIG. 17A, forcer 300 is provided with a rigidhousing 301 typically formed from machined or cast aluminum alloy.Housing 301 is provided with a face 302 with twelve pockets disposed inface 302 suitably sized to receive a set of twelve electromagnets 310 a,310 b, 310 c, 311 a, 311 b, 311 c, 312 a, 312 b, 312 c, 313 a, 313 b,and 313 c. The twelve electromagnets are constructed as in FIG. 10, FIG.12, and FIG. 13 or any of the possible variations previously described.Ribs such as 303 a and 303 b on face 302 form the pockets.Electromagnets such as 311 c are fixed within the pockets in face 302using high-strength epoxy or potting compound to provide preciserelative spacing.

A series of small orifice air-jets one of which is shown at 304 areprovided in face 302. An intersecting grid of holes, not shown, withinribs such as 303 b provide a means of connecting air-jets such as 304with a common source of compressed air. Not shown is a two dimensionalmagnetic platen consisting of square or rectangular teeth formed on aplanar surface by machining or etching two sets of orthogonal lines.

FIG. 17B shows planar motor 300 from the rear of case 301. Face 302 isnot visible. Case 301 is largely hollowed out on the rear, with ribssuch as 303 a and 303 b extending from face 302 a short distance intothe interior of case 301. The rear surfaces of electromagnets such as311 c are visible within the pockets formed by the ribs such as 303 a.Not shown are wire leads or a cable harness for connecting the windingsof electromagnets such as 311 c to some external amplifiers. A pneumaticfitting 305 is provided to couple compressed air to previously describedair-jets such as 304 of FIG. 17A. A number of threaded holes, one ofwhich is shown at 306, provide a means of attaching a cover or toolingplate, not shown, to case 301 for building machines incorporating planarmotor 300.

Operation—FIGS. 17A, 17B

Electromagnets 310 a, 310 b and 310 c are similar to electromagnets 206,208, and 210 of FIG. 10 and FIG. 12. Case 301 provides a means ofrigidly spacing electromagnets 310 a, 310 b, and 310 c apart from eachother in precise phase relationship as described for electromagnets 206,208 and 210 of FIG. 10 and FIG. 12. Case 301 and air-jets such as 304also provide a means of forming a very low friction air-bearing betweenplanar motor 300 and a toothed magnetic platen, not shown.

Electromagnets 310 a–c form a first linear motor capable of producingprecise force acting in the x-axis direction as shown by coordinates 307of FIG. 17A. Similarly, electromagnets 311 a–c form an identical secondlinear motor capable of producing precise force acting in the x-axisdirection of coordinates 307.

Electromagnets 312 a–c form a first linear motor capable of producingprecise force acting in the y-axis direction as shown by coordinates 307of FIG. 17A. Similarly, electromagnets 313 a–c form an identical secondlinear motor capable of producing precise force acting in the y-axisdirection of coordinates 307.

Electromagnets 310 a–c are offset in the y-axis direction of coordinates307 from electromagnets 311 a—c. Similarly, electromagnets 312 a–c areoffset in the x-axis direction of coordinates 307 from electromagnets313 a–c. This geometric relationship allows not only force production inthe x-axis and y-axis directions of coordinates 307 but also a limitedamount of rotation about a vector parallel to the z-axis direction aswell. The rotational range is typically +/−3 degrees. Rotational torqueproduction is only possible if the four linear motors consisting ofelectromagnets 310 a–c, 311 a–c, 312 a–c and 313 a–c are eachindependently controlled, typically from a 3-phase delta or Y-connectedamplifier, not shown.

Although it is possible to control planar motor 300 with open-loopstepper-motor controllers, it is now increasingly common to furtherequip case 301 with some means of position detection, not shown, inorder to provide highly precise closed-loop operation. Such positiondetection means are usually optical or magnetic sensors capable ofprecisely detecting position in x and y as well as rotationally about z.

Alternate Embodiments—FIGS. 14, 15, 16

Numerous variations of the above-described invention are possible tosuit particular requirements that may arise. The following examplesillustrate a few alternate embodiments that have merit.

FIG. 14 shows a phase vector diagram for obtaining 2-phase drive ofelectromagnets such as 206, 208, and 210 described in FIGS. 10 and 12.Phase vectors a, d, c′, f′, b, e, a′, d′, c, f, b′ and e′ are shownsimilarly to FIG. 13, however, they have been rotated clockwise by 15degrees for illustration. Six vectors, a, d, f′, b, e, and c, are chosenas primary phases and are illustrated with heavy lines. Two sourcephases, r and s, are 90 degrees apart as is typical for 2-phase drive,shown by dashed vectors. A first vector 260 is given by −pr while asecond vector 261 is given by −qs. Vectors 260 and 261 sum to producephase c from the drive phases r and s. For the vectors shown,p=q=0.7071. Equations for all phases are given as follows:a=mr−nsd=mr+nsf′=ms+nrb=ms−nre=qs−prc=−qs−pr

where m=0.9659, n=0.2588, p=0.7071, q=0.7071.

Each of the selected six phases, a, d, f′, b, e, and c are thus seen toconsist of a split phase winding similar to the case shown for FIG., 10,12, and 13. However, there are two different winding ratios needed.Phases a, d, f′, and b are similar, while phases e and c have adifferent winding ratio. Furthermore, the energy efficiency of thesephases is poorer than that of FIG. 13. The principle benefit of thisembodiment is for compatibility with existing 2-phase motor controllersand amplifiers. One skilled in the art will understand that there arenumerous phase winding ratios that can be chosen by rotating the vectorsr and s relative to phase vectors a, d, c′, f′, b, e, a′, d′, c, f, b′and e′.

FIG. 15 shows an alternate method of constructing phase windings on anelectromagnet 269, which is similar to electromagnet 206 of FIG. 12.Electromagnet 269 has a first magnetic core 270 coupled to a secondmagnetic core 272 by a permanent magnet 271. Core 270 is provided with afirst toothed pole 273 spaced apart from a second toothed pole 274 by aslot. Core 272 is provided with a first toothed pole 275 spaced apartfrom a second toothed pole 276 by a slot. The phase of pole 273 is 180degrees relative to the phase of pole 274, while the phase of pole 275is typically +/−30 degrees relative to pole 273. The phase of pole 275is 180 degrees relative to the phase of pole 276.

As was seen from the vector equations for FIG. 13, and the split phasewindings shown in FIG. 9 and FIG. 12, each pair of cores such as 211 and213 are provided with windings such as 214 a, 214 b, 215 a, and 215 baccording to the equations:a=mi−njd=mi−nk

where m=0.8165 and n=0.2988

Examination of the above equation indicates that three windings couldprovide the necessary split phases, namely a common winding of mi, forboth phase a and d, and two individual windings of nj, and nk. FIG. 15shows a winding 277 in the slot between pole 273 and pole 274. A commonwinding 279 is wound to encompass poles 274 and 275. A third winding 278is formed in the slot between pole 275 and pole 276. Winding 279provides mi, while winding 277 provides nj and winding 278 provides nk.

Electromagnet 269 of FIG. 15 provides better control of pole reluctancevariation, since winding 279 encompasses poles 274 and 275 which haveshort magnetic paths while independent windings 277 and 278 encompasspoles 273 and 276 which have longer magnetic paths. The number of turnsof winding 279 may be adjusted relative to the turns of windings 277 and278 to compensate for reluctance variation. However, the cost ofmanufacturing may be somewhat higher, and the thermal performance isslightly poorer relative to electromagnet 206 of FIG. 12.

FIG. 16 shows another alternate electromagnet configuration. Anelectromagnet 280 has a first U-shaped magnetic core 281 magneticallycoupled via a pair of permanent magnets 283 and 284, and a magnetic yoke285 to a second U-shaped magnetic core 282. Core 281 is provided with afirst salient toothed pole 286 that is spaced apart by a slot from asecond salient toothed pole 287. Core 282 is provided with a firstsalient toothed pole 288 that is spaced apart by a slot from a secondsalient toothed pole 289. A winding 290 is wound on pole 286 and on pole287. A winding 291 is wound on pole 288 and on pole 289. A commonwinding 292 is wound on poles 286, 287, 288, and 289. The phaserelationship of poles 286, 287, 288 and 289 is the same as the phaserelationship of poles 273, 274, 275, and 276 of FIG. 15.

Winding 292 provides the common mi term, while winding 290 provides njand winding 291 provides nk. Since each pole is encompassed by windingturns exclusively for one pole, the number of turns may be adjusted tocompensate for pole reluctance variation as noted for the design of FIG.15 as well. However, the overall size of the design of FIG. 16 is largerdue to windings protruding from the sides of poles 286 and 289.

The configuration of permanent magnets 283 and 284 along with magneticyoke 285 is that of the so-called back or exterior magnet design. Thiswell-known design does provide more equal pole reluctance values, asboth poles 286 and 287 as well as poles 288 and 289 are symmetricallyplaced with respect to magnets 283 and 284. However, the overall size ismuch larger than electromagnets of FIG. 12, and the design typically hashigher energy loss.

One skilled in the art will appreciate that additional combinations ofthe described embodiments may be useful. The exterior magnet structureof FIG. 16 may be used with the winding configurations of FIG. 12 andFIG. 15. The winding structure of FIG. 16 may also be used with thesandwich magnet structure of FIG. 12. A single permanent magnet such as283 or 284 may be used with yoke 285 shaped to close the magnetic pathas well. Various hybrid windings comprising elements shown in FIG. 12,FIG. 15, and FIG. 16 may be used to accomplish the same function.

CONCLUSION, RAMIFICATIONS, AND SCOPE

Accordingly, the reader will see that the Sawyer motor forcer of thepresent invention provides a number of advantages compared with existingsystems:

-   -   it improves the linearity of the motor force function by        providing a balanced 6-phase design, which has 50% more force        pulses per tooth pitch than existing 2/4-phase designs and 100%        more compared with existing 3-phase designs;    -   it maintains good magnetic path length stability in the        electromagnets, which improves motor force linearity;    -   it provides a uniform electromagnet winding configuration        allowing all electromagnets to be manufactured identically;    -   it enables 6-phase magnetic motors to be driven from standard        3-phase amplifiers and motor controllers, thus reducing the cost        of controlling linear or planar motors;    -   it reduces the number of electromagnets required from four to        three compared to the 2/4-phase designs, thus reducing costs and        enabling more compact motors.

Although the descriptions above provides detailed specification ofseveral presently preferred embodiments of this invention, these shouldnot be construed as limiting the scope of the invention. Countlessvariations and permutations of winding configurations with specificmagnet designs may be combined to produce alternate embodiments of thisinvention by one of ordinary skill in the art. Well known in the art arenumerous methods for utilizing such linear motors in various mechanicalframes in either single-axis systems or in dual-axis planar motorsystems. Multiple single-axis motors may be combined to form multi-axisrobotic or assembly machines for general purpose or specializedapplications. Although such linear motors generally employelectromagnetic structures with pole faces lying in a common plane, itis possible to use cylindrical surfaces, or multiple planes such asL-shaped, U-shaped, or rectangular tube shaped platens with some of theelectromagnetic poles disposed to face the various surfaces. The teethmay be formed in either of two directions on the pole surfaces of theelectromagnets, which merely changes the direction of travel relative tothe electromagnet structure. The specific geometry and size of teethspacing may vary by application, from a fraction of a mm to tens of cm.Accordingly, the scope of the present invention is limited only by theappended claims.

1. A six-phase linear motor forcer consists of: a set of electromagnets,of substantially similar construction, comprising a first electromagnet,a second electromagnet, and a third electromagnet, said firstelectromagnet being displaced proximate to said second electromagnet ina first direction, said second electromagnet being displaced proximateto said third electromagnet in said first direction, said firstdirection defining an axis of motion, said first electromagnet, saidsecond electromagnet, and said third electromagnet each furthercomprising a first magnetic core, said first magnetic core provided witha first salient pole having one or more teeth having a defined toothpitch Tp in said first direction, a second salient pole having one ormore teeth having a tooth pitch of said tooth pitch Tp in said firstdirection, and further including a winding slot disposed on said firstmagnetic core between said first salient pole and said second salientpole, said winding slot being provided with one or more coil windingsfor controllably altering the magnetic flux between said first salientpole and said second salient pole, the relative phase of said one ormore teeth of said first salient pole to the phase of said one or moreteeth of said second salient pole being approximately +/−180° of saidtooth pitch Tp in said first direction, a second magnetic core, saidsecond magnetic core provided with a first salient pole having one ormore teeth having a tooth pitch of said tooth pitch Tp in said firstdirection, a second salient pole having one or more teeth having a toothpitch Tp in said first direction, and further including a winding slotdisposed on said second magnetic core between said first salient poleand said second salient pole, said slot being provided with one or morecoil windings for controllably altering the magnetic flux between saidfirst salient pole and said second salient pole, the relative phase ofsaid one or more teeth of said first salient pole to the phase of saidone or more teeth of said second salient pole being approximately+/−180° of said tooth pitch Tp in said first direction, said secondsalient pole of said first magnetic core being displaced proximate insaid first direction to said first salient pole of said second magneticcore, said first magnetic core being magnetically coupled to said secondmagnetic core using a permanent magnet disposed between said secondsalient pole of said first magnetic core and said first salient pole ofsaid second magnetic core, wherein said set of electromagnets contains atotal of twelve toothed salient poles, further providing the relativephase alignment P of the teeth of any one salient pole to the teeth ofthe other eleven salient poles, specified in degrees of a said toothpitch Tp in said first direction is about P=+/−N×30°, where N=1, 2, 3,4, 5, 6, 7, 8, 9, 10, or 11, and further including a rigid housing forprecisely maintaining the relative spacing of said first electromagnet,said second electromagnet, and said third electromagnet so that said setof electromagnets may be easily and reliably incorporated into linear orplanar motor systems.
 2. The device of claim 1 wherein said winding slotof each of the magnetic cores of each of the electromagnets is providedwith a first coil winding and a second coil winding, said first coilwinding being electrically independent from said second coil winding foreffecting flux control in each of the magnetic cores of each of theelectromagnets from the sum or difference of two external electricalsources.
 3. The device of claim 2 wherein one of either said first coilwinding or said second coil winding of said first magnetic core iselectrically interconnected with one of either said first coil windingor said second coil winding of said second magnetic core on each of theelectromagnets wherefore each of the electromagnets is provided with afirst independent coil winding disposed on said first magnetic core, asecond independent coil winding disposed on said second magnetic core,and a third independent coil winding disposed commonly on both saidfirst magnetic core and said second magnetic core.
 4. The device ofclaim 3 wherein said third independent coil winding of said firstelectromagnet is electrically interconnected with one of either saidfirst independent coil winding or said second independent coil windingof said second electromagnet and is electrically interconnected with oneof either said first independent coil winding or said second independentcoil winding of said third electromagnet, said third independent coilwinding of said second electromagnet is electrically interconnected withone of either said first independent coil winding or said secondindependent coil winding of said first electromagnet and is electricallyinterconnected with one of either said first independent coil winding orsaid second independent coil winding of said third electromagnet, andsaid third independent coil winding of said third electromagnet iselectrically interconnected with one of either said first independentcoil winding or said second independent coil winding of said firstelectromagnet and is electrically interconnected with one of either saidfirst independent coil winding or said second independent coil windingof said second electromagnet, wherefore said set of electromagnets isprovided with a total of three electrically independent windings orphases to effect flux control of said set of electromagnets from anexternal three-phase electrical source.
 5. The device of claim 4 whereinthe relative phase alignment of said one or more teeth of said firstsalient pole of said first magnetic core to said one or more teeth ofsaid first salient pole of said second magnetic core is approximately+/−30° for each of said first electromagnet, said second electromagnet,and said third electromagnet.
 6. The device of claim 5 wherein saidthird independent coil winding on said first electromagnet is comprisedof a single common winding disposed in said winding slot of said firstmagnetic core and said winding slot of said second magnetic core of saidfirst electromagnet, said third independent coil winding on said secondelectromagnet is comprised of a single common winding disposed in saidwinding slot of said first magnetic core and said winding slot of saidsecond magnetic core of said second electromagnet, and said thirdindependent coil winding on said third electromagnet is comprised of asingle common winding disposed in said winding slot of said firstmagnetic core and said winding slot of said second magnetic core of saidthird electromagnet.
 7. The device of claim 4 wherein the relative phasealignment of said one or more teeth of said first salient pole of saidfirst magnetic core to said one or more teeth of said first salient poleof said second magnetic core is approximately +/−60° for each of saidfirst electromagnet, said second electromagnet, and said thirdelectromagnet.
 8. The device of claim 4 wherein the relative phasealignment of said one or more teeth of said first salient pole of saidfirst magnetic core to said one or more teeth of said first salient poleof said second magnetic core is approximately +/−90° for each of saidfirst electromagnet, said second electromagnet, and said thirdelectromagnet.